1. Field of the Invention
The invention relates to a medical image processing apparatus, a medical image processing system and a medical image processing method for performing an image process on a radiation image radiographed with a grid, the radiation image including a grid pattern.
2. Description of Related Art
Up to now, a radiation image represented by an X-ray image has been widely used for disease diagnosis or the like. These days, as a method for obtaining the radiation image, what is proposed is a radiation image recording/regenerating method with the use of “photostimulable phosphor”, which accumulates and records irradiated radiation energy, and which emits light according to the accumulated-recorded radiation energy when excitation light is irradiated thereto.
The radiation image recording/regenerating method is carried out in the following way. First, radiation which has transmitted through a subject is irradiated toward the photostimulable phosphor. Then, after radiation energy corresponding to radiation transmittance density of each part of the subject (hereafter, such radiation energy is referred to as “subject information”) is accumulated and recorded in the photostimulable phosphor, excitation light stimulates the accumulated-recorded radiation energy in the photostimulable phosphor to be emitted as light. Then, intensity of the emitted light is converted into electrical signals, and the electrical signals are transformed to regenerate a visible image in image recording material such as photosensitive material or the like, or in an image display device such as a CRT or the like.
However, according to the radiation image recording/regenerating method, although there is an advantage that it is possible to obtain a radiation image having abundant information amount, with extremely low exposure dose compared to a radiography method with the use of silver halide photosensitive material, the photostimulable phosphor also accumulates and records low-energy radiation which is a result of scattering the radiation which is transmitted through the subject (scattered radiation), due to its highly sensitive characteristic. The scattered radiation may prevent from accumulating and recording subject information accurately, and thereby there is a possibility of causing various bad effects such as decrease of diagnosis efficiency.
As a way of eliminating scattered radiation, what is used is “grid”, which is a laminated member covered with a cover member having low radiation absorption rate, in which radiation absorption layers made of lead or the like having high radiation absorption rate, and radiation transmittance layers made of aluminum, paper, wood, synthetic resin or the like having low radiation absorption rate are alternately laminated. The grid is located in front of the photostimulable phosphor in order to eliminate scattered radiation.
However, as described above, since the radiation absorption layers and the radiation transmittance layers are laminated alternately, a striped pattern corresponding to the grid (grid pattern) appears on the radiation image as a consequence. Based on this problem, what is known is an image processing method which is capable of eliminating moire based on an interference pattern by applying a filtering process on an original image with the use of a filter having a characteristic to eliminate the grid pattern, at the time of performing an image processing by performing multiple resolution conversion on an image including the grid pattern (see Japanese Patent Application Publication (Unexamined) No. Tokukaihei 9-44645).
Further, in a cyclical pattern suppressing method for suppressing spatial frequency component of a cyclical pattern caused from a static grid or the like, what is known is a cyclical pattern suppressing process method which is capable of obtaining a sharp, high quality image without visible appearance of a striped pattern caused from a static grid, by applying the two-dimensional wavelet transform with the use of the low-pass filter, which has a characteristic under which there is approximately no response at not less than spatial frequency corresponding to a grid pitch, for suppressing only a predetermined range adjacent to the spatial frequency component of the static grid (see Japanese Patent Application Publication (Unexamined) No. Tokukai 2001-273490).
Alternately, in the case of regenerating image data which is obtained by reading an image radiographed with a grid having different pitches, what is known is an image processing method which is capable of reducing a striped pattern caused from a grid having any pitch, from image data on which the filtering process is applied, by using a moire eliminating filter capable of reducing response of not less than 97% of spatial frequency component with respect to the special frequency component corresponding to the grid pitch, down to not more than 5% (see Japanese Patent Application Publication (Unexamined) No. Tokukai 2000-3440).
By the way, in the above-mentioned radiation image recording/regenerating method, if a normal grid having an even grid pitch is used, due to influence of an irradiation angle of a radiographing apparatus (X-ray bulb), a pitch of a grid pattern projected on a detector such as the photostimulable phosphor, an FPD or the like does not become even. For example, as shown in FIG. 7, X rays irradiated from an X-ray source 2 are transmitted through lead foils 3a and 3b of a grid 3, and then projected on an image receiving screen of a detector 4 as images 4a and 4b. Here, due to the influence of an irradiation angle of a bulb, the image 4a of the lead foil 3a is projected as a wider image than the image 4b of the lead foil 3b, which is located in a direction of primary X-ray. As a result, a grid pattern projected on the detector 4 has a characteristic of having narrow pitch at the center part but having wider pitch as the location is closer to the end part.
However, only one kind of filter is used in the conventional image processing method, which either eliminates or reduces a striped pattern caused from a grid. Therefore, for example, if a filter characteristic is determined based on image data proximate to a position perpendicular to the bulb (for example, at the center part of the detector), it is not possible to demonstrate the favorable filter characteristic around the end part where the bulb has a predetermined irradiation angle, and thereby there is a problem of decreasing performance to eliminate a grid pattern.
Further, if a focused grid is used, although an improvement can be seen on a difference of a pitch in a grid pattern between the center part and the end part, it is not possible to entirely eliminate the difference of a pitch in the grid pattern. Further, since unevenness of the pitch may occur at the grid production, and/or since unevenness of pitch may occur according to relative positions of the grid and the detector at radiography, it is difficult to eliminate a grid pattern which evenly covers the entire width of the detector.